Phosphorus deficiency

Stunted plants with a dull purple cast to the older leaves, slow to flower and slower to fruit. Usually cold soil rather than empty soil.

Disorder Serious Doesn't spread

What you'll see

A dull, dark blue-green color to the whole plant rather than a healthy fresh green.
Purple or reddish tints on the undersides and stalks of the oldest leaves, and on the stems.
Growth stunted and slow, with small leaves and short internodes.
Flowering and fruiting late and sparse, and roots poorly developed.
Sweetcorn seedlings purple-striped in a cold spring and growing out of it as the soil warms — the commonest version by far.

Why it happens

Phosphorus barely moves in soil, so a root has to grow to it rather than the other way round. Most garden soils have plenty; what stops the plant getting it is cold soil, which slows root growth and uptake, or a pH outside about 6.0 to 7.0, which locks it up chemically — alkaline soil binds it to calcium, acid soil to iron and aluminum.

Cold wet springs, compacted soil, very acid or very alkaline soil, and root damage. Young plants show it and mature ones rarely do, because by then the roots have found what they need.

Treating it

Test the soil before adding anything. Excess phosphorus is a real problem in gardens that have been manured for years, and it locks up iron and zinc in its turn.
Wait for the soil to warm if the plants are young and the weather has been cold; most cases correct themselves.
Correct the pH, which is far more often the answer than adding phosphorus: lime an acid soil, add sulfur or organic matter to an alkaline one.
Use bone meal, rock phosphate or a balanced organic fertilizer worked into the root zone if a test really shows a shortage.
Foliar-feed with a soluble phosphorus source for a quick correction while the soil fix takes effect.

Keeping it away

Test the soil every three or four years rather than feeding blind.
Warm the soil before sowing or planting with a cloche, fleece or black plastic.
Keep the pH between 6.0 and 7.0 for most vegetables, which is where phosphorus is most available.
Add compost and encourage mycorrhizal fungi, which extend the root system enormously and are the natural answer.
Do not add phosphorus routinely; it accumulates, it does not leach, and the excess runs off into watercourses.

Who gets it

Sweetcorn, tomatoes, brassicas and young transplants of all kinds show it most clearly.